Transmitter circuitry transmits: a first voltage as the return-to-zero signal that is higher than a first positive threshold, the first voltage being decodable to a first order of data bits; a second voltage as a return-to-zero signal that is between a second positive threshold and the first positive threshold, the second voltage being decodable to a second order of the data bits, and the second positive threshold being lower than the first positive threshold; a third voltage as the return-to-zero signal that is between a first negative threshold and a second negative threshold, the third voltage being decodable to a third order of the data bits, and the second negative threshold being higher than the first negative threshold; and a fourth voltage as the return-to-zero signal that is lower than the first negative threshold, the fourth voltage being decodable to a fourth order of the data bits. Clock circuitry transitions a clock signal for the return-to-zero signal crossing the second positive threshold, and for the return-to-zero signal crossing the second negative threshold.